Tungsten Carbide Films Research Articles

Preparation of Tungsten Carbide Films by Pulsed Vacuum Arc Deposition

Tungsten carbide films were synthesized on Si wafer by pulsed vacuum arc deposition using a sintered tungsten carbide cathode. Arc discharge voltage, capacitance of capacitor, and cathode-substrate distance were controlled for changing film properties. For the evaluation of crystal structure, X-ray diffraction analysis was performed. Mechanical properties of films were characterized using AFM, nano-indentation and ball-on-disc friction tests, and the results were compared to those of films deposited by R. F. magnetron sputtering. The hardness of films was in the range of 9-28 GPa. Friction coefficients were almost the constant values of 0.4-0.5. Harder film showed lower wear rate. Films deposited by pulsed arc deposition were smoother and high wear-resistant.

Relatively low temperature synthesis of hexagonal tungsten carbide films by N doping and its effect on the preferred orientation, phase transition, and mechanical properties

The authors deposited N-doped tungsten carbide thin films on Si(100) substrates at 500°C using direct-current reactive magnetron sputtering in a mixture of CH4∕N2∕Ar discharge and explored the effects of N doping on the preferred orientation, phase transition, and mechanical properties of the films by using x-ray diffraction, x-ray photoelectron spectroscopy, and nanoindentation measurements. They found that N doping significantly influenced the compressive stress, which led to a pronounced change in the preferred orientation, phase structure, and hardness for the tungsten carbide film. A phase transition from β-WC to α-WC occurred when N doping was in the range of 2.9 and 4.7at.%, meaning that α-WC can be obtained at relatively low temperature (500°C). To reveal the relationship between the stress and phase transition, as well as preferred orientation, the density-functional theory based on first principles was used to calculate the elastic constants and shear modulus for tungsten carbide with a structure of β-WC or α-WC. The calculated results showed that the preferred orientation depended on the competition between strain energy and surface energy, as well as the grains competitive growth, and the phase transition can be attributed to a decrease in the strain energy. The hardness of α-WC was harder than β-WC because the shear modulus for α-WC was larger than that of β-WC, whereas the bulk modulus for α-WC was almost equal to that of β-WC.

Chemical deposition of nanostructured tungsten and tungsten-alloy coatings from gas phase

The historical overview of the fluoride method for the low-temperature deposition of tungsten, its alloys, and compounds in Russia is presented. The physicochemical fundamentals (thermodynamics, kinetics) of the deposition process are given. The most probable rout for homogeneous and heterogeneous reactions resulting in the formation of the tungsten metal at the substrate surface is theoretically substantiated. Based on the experimental study of the initial crystallization stages, a basic mechanism of the active growth centers formation is formulated. Kinetic characteristics of the processes of tungsten codeposition with refractory transition metals, as well as the coatingsí physical-mechanical properties are given. Data on the deposition of solid tungsten-carbide films from gas phase onto low-temperature substrate are reported. Possible applications of the tungsten, tungsten alloys with transition metals, and tungsten carbide coatings are discussed.

Use of diamond-like carbon with tungsten (W-DLC) films as biocompatible material

Diamond-like carbon (DLC), also known as amorphous hydrogenated carbon (a-C:H), are a class of materials with excellent mechanical, tribological and biological properties. When the DLC films are enhanced with other elements, all of these properties can be changed within a certain range. In this work, reactive magnetron sputtering was used to deposit W-DLC (hydrogenated tungsten carbide) films on Ti6Al4V (implant material). Many films were made using pure tungsten (99.99%) target and different plasmas processes, with different ratio among argon and methane. It was possible to change the films composition (from pure amorphous carbon to carbon enhanced with tungsten) according to ratio of argon and methane plasma. Between all films processed, the carbon films enhanced with tungsten showed good results in the “in vitro” cytotoxicity testing. Raman spectroscopy was used to analyze the chemical bonds kinds and the chemical bonds quantities. The Rutherford Back Scattering (RBS) was used to analyze the films compositions. The chemical inertness was analyzed by scanning voltametry. W-DLC thin films obtained in these processes have low roughness, high chemical resistance, good adhesion and show a high biocompatibility, when compared with common DLC thin films. Hence we have concluded that the tungsten concentrations in the DLC films make an important role to improve the properties of the DLC layers.

Characterization of W1Cx electrical contacts on silicon carbide using RBS and AFM/SEM

In this work we are investigating the structural evolution of tungsten carbide films deposited on silicon carbide (SiC) substrates as a function of temperature. We used thermal evaporation and tungsten carbide (WC) powder as a starting material to produce the thin layer deposition on semi-insulating silicon carbide (6H). Our intended applications are for devices working at 800°C; therefore, our investigations are carried out at 1h intervals of time the samples spent at this temperature. We used Rutherford backscattering spectrometry (RBS) for measuring the stoichiometry and depth profile, atomic force microscopy (AFM) and scanning electron microscopy (SEM) to monitor the surface morphology change, and optical measurements (Raman and optical absorption spectroscopy) to monitor the eventual apparition and evolution of chemically-induced defects of the SiC crystalline lattice.

Structural and chemical phase transitions in tungsten carbide films evidenced by the analysis of their stiffness tensors

Brillouin light scattering (BLS) is used to provide a comprehensive study of thin tungsten carbide films deposited on single crystal silicon substrates whose distinctive nature depends critically on the deposition parameters. The use of stepped films in these slow-on-fast systems provides enhanced data sets and allows the velocity dispersion of the observed surface excitations, including the discrete Rayleigh and Sezawa modes to be studied in detail. Comprehensive and powerful methods of data analysis and interpretation including the recently developed Monte Carlo (MC) method, the surface Green’s function, and classical approaches are applied to extract the effective elastic constants and density of each of the films. The MC and Green’s function methods are used to remove ambiguities in Sezawa mode assignments and to identify a mode-crossing event. Auger electron spectroscopy and x-ray diffraction investigations confirm conclusions about chemical composition and microstructure obtained by BLS including a structural phase transition, thus leading to a consistent description of elastic, structural, and chemical properties of tungsten carbide films as a function of their deposition conditions. The anisotropic elastic tensors of the various films are employed for an analysis of the angular dependent Young’s modulus and the shear modulus, suggesting implications for the film performance in wear protection. Finally, an estimate of the elastic anisotropy of the α-W2C single crystal is provided on the basis of the effective elastic constants of a nanocrystalline W2C film.

Microstructural and mechanical investigations of tungsten carbide films deposited by reactive RF sputtering

Tungsten carbide thin films were deposited on (100) Si single crystal substrates by reactive RF sputtering from a tungsten target in Ar–CH 4 mixture. Under 1% of methane, the formation of a mixture of nanocrystalline WC 1− x presenting a partial (200) texture and W 2C phases was observed. The film deposited under 2% of CH 4 shows a maximum hardness of 22 GPa (a value comparable to the bulk WC reference material), these coatings also exhibit the lowest friction coefficient of 0.1. For coatings performed with CH 4 percentage higher than 3%, a progressive amorphization of the layers was observed. At 5% of CH 4 concentration, the films become essentially amorphous. Simultaneously the hardness decreases continuously to achieve a minimum value of 13 GPa.

Reactive Sputter Deposition of WC<sub>x</sub> Films for Replacing Hexavalent Chromium

This paper investigates the reactive sputter deposition of tungsten carbide (WCx) films for replacing hexavalent chromium which was found to cause cancer in human body. The deposition rate of the films was proportional to rf-power and inversely proportional to the CH4 content in the sputtering gas. The hardness of the WCx coatings increased as the rf-power increased. The highest hardness was obtained at the CH4 concentration of 10 vol.% in the sputtering gas. The hardness of the WCx film deposited under optimal conditions was far higher than that of the electroplated chromium film although the corrosion-resistance of the former was slightly inferior to that of the latter. Considering all these experimental results, it may be concluded that the reactive sputterdeposition of WCx coatings can be effectively applied to the parts with complex geometries as an alternative to chromium electroplating.

Effects of Process Parameters on the Deposition Rate, Hardness, and Corrosion Resistance of Tungsten Carbide Coatings Deposited by Reactive Sputtering

The reactive sputter deposition of tungsten carbide (WCx) films as an alternative to chromium electroplating was studied. The effects of rf power, pressure, sputtering gas composition, and substrate temperature on the deposition rate of the WCx coatings were investigated. The effects of rf power and sputtering gas composition on the hardness and corrosion resistance of the WCx coatings were also investigated. X-ray diffraction (XRD) and Auger electron spectroscopy (AES) analyses were performed to determine the structures and compositions of the films, respectively. The hardnesses of the films were measured using a nanoindenter. The microstructures of the films were observed by scanning electron microscopy. The corrosion resistances of the films were evaluated using a salt-spray test. The deposition rate of the films was proportional to rf power and inversely proportional to the CH4 content of the sputtering gas. The deposition rate increased linearly with increasing chamber pressure. The hardness of the WCx coatings increased as rf power increased. The highest hardness was obtained at a CH4 concentration of 10 vol.% in the sputtering gas. The hardness of the WCx film deposited under optimal conditions was much higher than that of the electroplated chromium film, although the corrosion resistance of the former was slightly lower than that of the latter.

Synthesis, characterization and surface reactivity of tungsten carbide (WC) PVD films

Phase-pure tungsten carbide (WC) and Pt-modified WC thin films have been synthesized using the physical vapor deposition (PVD) technique. The physical and chemical properties of the PVD films have been studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and near edge X-ray adsorption fine structure (NEXAFS). TPD and HREELS have been utilized to investigate the reactivity of the WC and Pt-modified WC films toward the reaction with cyclohexene. The addition of Pt on the WC surface enhances the selective dehydrogenation pathway to produce gas phase benzene. The results reported here illustrate the feasibility of utilizing TPD and HREELS to investigate the surface chemistry of well-characterized PVD films.

The effects of substrate bias on phase stability and properties of sputter-deposited tungsten carbide

The effects of substrate bias on phase formation and physical properties of rf magnetron sputter-deposited tungsten carbide films have been investigated in this work. Films were deposited at 275 °C using bias levels ranging from 0 to −150 V and at room temperature and −160 V. At low bias levels, the films were primarily composed of the WC 1− x /W 2(C,O) phases, both of which have the B1 structure, and the fraction of the hexagonal W 2C phase increased with bias level. The increased substrate bias levels also correlated with a reduction in oxygen content, suggesting that reducing oxygen content promotes formation of the W 2C phase. However, the film deposited at room temperature and −160 V bias had an oxygen content of only 3%, yet did not form the W 2C phase, indicating a minimum level of thermal activation is also required to form W 2C. Increasing the bias voltage also resulting in increases in film hardness, modulus and compressive residual stress, while reducing resistivity.

The use of guided surface acoustic resonances in the determination of the thin film elastic tensor

A Brillouin light scattering investigation of thin supported films shows that the measurement of resonances within the continuum of excitations together with the dispersion of discrete surface acoustic modes such as the Rayleigh and Sezawa waves, yields valuable additional information. In the present study, an analysis of the combined data results in the precise determination of four independent components of the film elastic tensor of elastically anisotropic, polycrystalline tungsten carbide (W2C) films for which the single crystal elastic constants are not known.

The role of contaminants in the variation of adhesion, friction, and electrical conduction properties of carbide-coated scanning probe tips and Pt(111) in ultrahigh vacuum

Scanning probe microscopy was used to investigate the tribological properties of nanoscale tips in contact with a Pt(111) single-crystal surface under ultrahigh vacuum conditions. The tips were coated with a tungsten carbide film, which contained a significant fraction of oxygen. The electrically conductive tip made it possible to alternate between contact measurements and noncontact scanning tunneling microscopy. Several types of interfaces were found depending on the chemical state of the surfaces. The first type is characterized by strong irreversible adhesion followed by material transfer between tip and sample. Low adhesion and no material transfer characterize a second type of contact, which are associated with the presence of passivating adsorbates in both (full passivation) or in one of the two contacting surfaces (half-passivation). Half-passivated contacts in which the clean side is the Pt(111) sample gave rise to periodic stick-slip friction behavior with a period equal to the atomic lattice constant of the Pt(111) surface. Local electrical conductivity measurements show a clear correlation between electronic and friction properties, with ohmic behavior on clean regions of the Pt surface and semiconductor-like behavior on areas covered with adsorbates.

General methods for the determination of the stiffness tensor and mass density of thin films using Brillouin light scattering: Study of tungsten carbide films

Methods of film preparation and data evaluation are presented to determine the stiffness tensor and mass density of thin films using Brillouin light scattering from surface phonons. The combined use of stepped films of progressively increasing thickness $d,$ and an optimized variation of the magnitude of the surface wave vector ${q}_{\ensuremath{\parallel}}$ provides a particularly wide range of uniformly distributed values of the product ${q}_{\ensuremath{\parallel}}d$ in the study of the velocity dispersion of the observed surface acoustic waves. Furthermore by the independent variation of ${q}_{\ensuremath{\parallel}}$ and $d,$ the microstructure can be assessed as a function of film thickness. In the present case of tungsten carbide (WC) films on a silicon substrate, being a slow-on-fast system, the Rayleigh mode and several Sezawa modes are observed. Using the extensive velocity dispersion data, five independent material parameters of the WC films, i.e., four elastic constants and the mass density, are determined by two independent approaches. One employs a least-squares fit to the experimental velocity dispersion curves to determine these material parameters, whereas the other investigates the structure of the parameter space in the vicinity of the solution being sought by a Monte Carlo technique. Both approaches yield identical and self-consistent results. In a Green's-function formulation that uses the vertical displacement component of the acoustic excitations at the free surface, the experimental Brillouin spectra are simulated in the discrete as well as in the continuous part of the mode spectrum.

Spectroscopic ellipsometry (SE) and grazing X-ray reflectometry (GXR) analyses on tungsten carbide films for diffusion barrier in copper metallization schemes

Tungsten carbide (WC x , x∼0.5) films, for diffusion barrier applications in copper metallization schemes, were prepared by a pulsed chemical vapor deposition (PCVD). Spectroscopic ellipsometry (SE) and grazing X-ray reflectometry (GXR) were used to characterize WC x films for thickness, surface roughness, and their optical properties. It is found that Drude law with two Lorentz absorption bands could be used to describe the optical dispersion behaviors of WC x films. Structural and chemical characterization of the films was performed with Auger electron spectroscopy (AES), X-ray diffraction spectroscopy (XRD), and atomic force microscopy (AFM). Surface roughness obtained from GXR and SE is consistent with AFM measurements.

Epitaxial growth of tungsten carbide films using C 60 as carbon precursor

In this paper we present the first results from processing and characterisation of epitaxial tungsten carbide films. The films were deposited in ultra-high vacuum by DC magnetron sputtering of tungsten with co-evaporated C 60 as carbon source. It was found that the growth rate was crucial for the crystalline film quality. Epitaxial growth of β-WC 1− x could be achieved at 400°C using a very low deposition rate of 6 Å/min. Single-crystal films were thus formed on both MgO(1 0 0) and MgO(1 1 1). High-resolution transmission electron microscopy shows that the films grow with a very good match relative the substrate. Deposition rates of 10 Å/min and higher resulted in renucleation and nanocrystalline growth with grain sizes of 15–80 Å depending on substrate temperature. We have also found that a small addition of titanium stabilises the epitaxial growth of the cubic tungsten carbide films at higher growth rates. A titanium contribution of ∼2% give epitaxial growth at deposition rates of 40 Å/min. X-ray diffraction study of the epitaxial films using reciprocal space mapping shows a cross-like broadening that is possibly related to ordering.

Magnetron sputtered W–C films with C 60 as carbon source

Thin films in the W–C system were prepared by magnetron sputtering of W with coevaporated C 60 as carbon source. Epitaxial deposition of different W–C phases is demonstrated. In addition, nanocrystalline tungsten carbide film growth is also observed. At low C 60/W ratios, epitaxial growth of α-W with a solid solution of carbon was obtained on MgO(001) and Al 2O 3(001) at 400 °C. The carbon content in these films (10–20 at.%) was at least an order of magnitude higher than the maximum equilibrium solubility and gives rise to an extreme hardening effect. Nanoindentation measurements showed that the hardness of these films increased with the carbon content and values as high as 35 GPa were observed. At high C 60/W ratios, films of the cubic β-WC 1− x ( x=0–0.6) phase were deposited with a nanocrystalline microstructure. Films with a grain size <30 Å were obtained and the hardness of these films varied from 14 to 24 GPa. At intermediate C 60/W ratios, epitaxial films of hexagonal W 2C were deposited on MgO(111) at 400 °C. Polycrystalline phase mixtures were obtained on other substrates and hexagonal WC could be deposited as minority phase at 800 °C.

Structure and mechanical properties of tungsten carbide films deposited by off-plane double bend filtered cathodic vacuum arc

High quality nanocrystalline tungsten carbide films were deposited by a filtered cathodic vacuum arc technique at room temperatures. X-ray diffraction and atomic force microscopy were used to characterize the crystalline structure and surface morphology of the films. Substrate bending methods and nanoindenter were used to measure internal stress, hardness, and Young’s modulus of the films. The film structure evolves from a single hexagonal α-W2C phase to a mixture phase, which is composed of a hexagonal α-W2C, a cubic WC1−x, and a cubic W phase, as the substrate bias, which is always negative, is increased to 200 V. As the substrate bias is further increased, the film structure changes gradually to a single α-W2C phase again. The variation trends of internal stress, hardness, and Young’s modulus with the substrate bias are similar. They increase with substrate bias, reaching a maximum of 7.2, 26, and 270 GPa, respectively, at a substrate bias of 200 V, then decrease with further increase of substrate bias.

Effects of carbon incorporation in tungsten carbide films deposited by r.f. magnetron sputtering: single layers and multilayers

Tungsten carbide coatings with different carbon content have been obtained by r.f. magnetron sputtering from a sintered pure WC target on high-speed steel. Two different sputtering deposition processes were tried in order to deposit tungsten carbides with two stoichiometries: one with pure Ar, and the other one with a reactive Ar/CH4 mixture. Single tungsten carbide coatings obtained with pure Ar show a cubic WC1−x phase with a strong (200) crystallographic orientation, while single coatings deposited with the reactive mixture of gases show a (111) preferred orientation. Multilayer structures were also deposited by sequentially changing the sputtering gas composition between both single layer deposition conditions. The bilayer period was thus varied from 100 to 10 nm. SIMS analysis confirmed the coatings multilayer structure. XRD analysis of the multilayers shows a combination of the orientations obtained for single layers with a progressive decrease of (200) orientation when decreasing the bilayer period. Nanoindentation, microscratch and ball-on-disk tests were performed over all the coatings to compare their mechanical and tribological properties.

Superconducting fluctuation magnetoconductance in a tungsten carbide film

Magnetoconductance (MC) measurements have been performed on a 2140 Å thick tungsten carbide film at temperatures very close to the superconducting transition temperature Tc of the film. The data are dominated by superconducting fluctuations. A novel three-dimensional phenomenological model is proposed to explain the MC data, yielding good fits. The Larkin beta factor, βLarkin, appeared as a fitting parameter. An expression proposed by Larkin for βLarkin failed badly for temperatures extremely close to Tc. But at intermediate and high temperatures compared to Tc, Larkin's expression gave very good agreement in fits to the MC data. At temperatures very close to Tc, a crossover from three dimensions to two dimensions was observed in the behaviour of the MC of the film.